Today there is an ever increasing demand for soft, bulky tissue products, which also have sufficient tensile strength to withstand use. Traditionally the tissue maker has solved the problem of increasing sheet bulk without compromising strength and softness by adopting tissue making processes that only minimally compress the tissue web during manufacture, such as through-air drying. Although such techniques have improved sheet bulk, they have their limitations. For example, to obtain satisfactory softness the through-air dried tissue webs often need to be calendered, which may negate much of the bulk obtained by through-air drying.
Tissue product bulk may also be increased by treating a portion of the papermaking furnish with chemicals that facilitate the formation of covalent bonds between adjacent cellulose molecules. This process, commonly referred to as cross-linking, often involves the reaction of water soluble multi-functional molecules capable of reacting with cellulose under mildly acidic conditions. The cross-linking agents are generally methylol or alkoxymethyl derivatives of different N-containing compounds such as urea and cyclic ureas. Polycarboxylic acids and citric acid have also been used with varying degrees of success. Sheets formed from cross-linked cellulosic fibers, while having increased bulk, generally have poor tensile and tear strength, because of reduced fiber to fiber bonding.
To lessen the negative effects of cross-linked fibers the prior art has resorted to alternative cross-linking agents and to blending cross-linked and uncross-linked fibers together. For example, in U.S. Pat. No. 3,434,918 sheeted fiber is treated with a crosslinking agent and catalyst and wet aged to insolubilize the crosslinking agent. The fiber sheet is then dispersed and blended with non-cross-linked fibers to form a fiber slurry used to form a creped tissue web, which is subsequently passed under a dryer to cure the crosslinking-agent. In U.S. Pat. No. 3,455,778 bleached southern softwood kraft pulp is reacted with dimethylol urea to form cross-linked fibers, which are blended with untreated hardwood and softwood pulps. The blended pulps were used to form a creped tissue web having improved absorbent properties. In U.S. Pat. No. 4,204,054 wood pulp fibers were sprayed with a solution of formaldehyde, formic acid and hydrochloric acid and then immediately dispersed in a hot air stream for 1-20 seconds to form cross-linked fibers. The cross-linked fibers were then blended with uncross-linked fibers to form a sheet having improved flexibility and water absorbency. Finally, in U.S. Pat. No. 6,837,972 cross-linked cellulosic fibers are blended with softwood kraft pulps having an elevated hemicellulose content to form tissue webs. The tissue webs, while having increased bulk, have greatly diminished tensile strength.
Accordingly, what is needed in the art is a tissue product comprising cross-linked fibers that is both bulky and strong without any decrease in softness.